Overcurrent proof constant voltage

ABSTRACT

A power source circuit having an overcurrent preventing means comprising a transistor connected in series with a load for controlling the output voltage supplied to the load to a constant value, and a base circuit for the transistor including a thyristor triggered by a voltage produced across a current detection resistor connected in series with the load and a delay means to cut off the thyristor a predetermined time after the triggering of the thyristor. The delay means consists of either a combination of a resistor and a capacitor or a combination of a lamp and a photoconductive cell. The thyristor is short circuited either indirectly by driving another switching means or directly through the main component element of the delay means to cut off the thyristor.

United States Patent Matsushima et al.

[54] OVERCURRENT PROOF CONSTANT- VOLTAGE [72] Inventors: HiroshiMatsushima, Osaka; lchiro Arlmura, Kyoto; Hiroshi Goto, Osaka; Yoshikad-[21] Appl. No.: 68,719

[ Feb. 15, 1972 3,496,415 2/1970 Ruthenberg ..3l7/33 VR X 3,538,42611/1970 Jones l ..323/9 3,303,388 2/1967 Means ..3l7/33 VR PrimaryExaminer-J. D. Miller Assistant Examiner-Harvey FendelmanAttorneyStevens, Davis, Miller 8!. Mosher [57] ABSTRACT A power sourcecircuit having an overcurrent preventing 30 F A It ti n m Data meanscomprising a transistor connected in series with a load 1 orelgn pp ca 0o ly for controlling the output voltage supplied to the load to a Sept.17, 1969 Japan ..44/750l3 constant value, and a base circuit f thetransistor induding a Sept. 18, 1969 Japan ..44/75586 thyristortriggered by a voltage pt-oducgd across a current tection resistorconnected in series with the load and a delay [52] US. Cl ..323/9,323/20, 317/33 VR, means to cut off the thyristor a predetemtined timeafter the 317/33 2 4 5 triggering of the thyristor. The delay meansconsists of either a 2: 36 combination of a resistor and a capacitor ora combination of l l e o are 323/9 307/252 J a lamp and aphotoconductive cell. The thyristor is short circuited either indirectlyby driving another switching means or [56] References Cited directlythrough the main component element of the delay means to cut off thethyristor. UNITED STATES PATENTS 3,448,342 6/1969 Jacobs ..3l7/33 5Claims, 6 Drawing Figures 0 W3 Act L/he t W? EV 3 L 02 T Pmmmm 15 m23.643.151

SHEET 1 OF 3 AC L/n g L AC. Line fl W/MWMIW/W flwfd 4W0 X M1690INVENTORY ATTORNEY PATENIEBFEB 15 I972 3.643.151

SHEET 2 BF 3 4C L/he AC M785 OVERCUR-RENT PROOF CONSTANT VOLTAGE Thisinvention relates to power source circuits and, more particularly, topower source circuits having an overcurrent preventing means.

The power source supplying current to a load sometimes supplies excesscurrent to the load due to an accident (for instance, a short circuit inthe load). Excess current caused through the load is burdensome not onlyfor the load, but also for the power source. If such excess currentcontinues to flow, the power source would be damaged. To avoid thecontinued flow of the overcurrent, the power source circuit'is usuallyprovided with an overcurrent preventing means (aprotection circuit).With the power source circuit having a thyristor, the thyristor onceturned on by being triggered due to excess current accidentally causedthrough the system remains conductive. In order to operate the powersource circuit in the normal state again, a circuit to turn off thethyristor is required. Usually, a manual switch means is used to the endof turning off the thyristor.

According to the invention, the protection circuit having a thyristor isprovided with a means to automatically restore the normal operativestate of the power source circuit, so that the power source circuit maybe automatically returned to the nonnal operative state a constant timeafter it is rendered into a current-withholding state.

Accordingly, an object of the invention is to provide a power sourcecircuit provided with an overcurrent preventing means of anautomatically restorable type.

Another object of the invention is to provide for a constant withholdingperiod from the actuation of the overcurrent preventing means until therestoration of the normal state of the power source circuit.

These and other objects and features of the invention will become moreapparent from the following description with reference to theaccompanying drawing, in which:

FIG. 1 is a circuit diagram of a typical example of the prior artconstant-voltage power source circuit;

FIG. 2 is a circuit diagram of a prior art constant-voltage power sourcecircuit provided with an overcurrent preventing means using a thyristor;

' HO. 3 is a circuit diagram of a power source circuit provided with anautomatically restorable type overcurrent preventing means embodying theinvention;

FIG. 4 is a circuit diagram of another embodiment of the power sourcecircuit having an automatically restorable .type overcurrent preventingmeans according to the invention;

FIG. 5 is a graph illustrating the operational principles underlying theinvention; and v FlG. 6 is a graph illustrating the operationalprinciples involved in the embodiment of FIG. 4.

FIG. 1 shows a fundamental constant-voltage circuit. It comprisesrectifying diodes D, and D a smoothing capacitor C,, a smoothing circuitincluding a resistor R, and a capacitor C a current control transistorTr,, an error voltage amplifier consisting of a transistor Tr a seriescircuit including a zener diode D and a resistor R to provide a constantvoltage to the emitter of the transistor Tr and a variable resistor VRto provide a preset output voltage. Reference symbol L designates a loadconnected to the constant-voltage circuit.

In the operation of the constant voltage just described, when theterminal voltage across the load L exceeds a predetermined value, thepotential difference between the voltage drop across part of thevariable resistor VR, which is fed to the base of the transistor Tr andthe terminal voltage across the zener diode D, is impressed on the baseof the transistor Tr, to decrease the base potential thereof. As aresult, the conduction level of the transistor Tr, is reduced todecrease the output voltage supplied across the terminals of the load L.On the other hand, when the voltage supplied to the load L becomes lowerthan the predetermined voltage, the conduction level of the transistorTr, is raised to increase a current into the load L, thus increasing theterminal voltage thereacross.

lf the load L is short circuited or rendered into a substantiallyshort-circuited state so that the load current becomes higher than apredetermined value, the collector power dissipation in the transistorTr, is increased. If this situation continues for a long time, thetransistor Tr, would be thermally broken. To avoid this danger, therehave been proposed various circuits, which can avoid overcurrent.

FIG. 2 shows one such circuit using a thyristor. In this circuit,overcurrent is detected by a resistor R,. The resistor R, is selectedsuchthat'when the voltage drop thereacross exceeds a predetermined valuea thyristor D is turned on. 'When the thyristor D is turned on, it shortcircuits the base of the transistor Tr, to render the emitter potentialof the transistor Tr, nearly zero, thus substantially cutting currentoff the load L. With this circuit, there is no possibility of thethermal breaking of the transistor Tr,. In this circuit, when thethyristor D is once turned on, it continues to carry current, which ischaracteristic of the circuit using a thyristor. In order to recover theoriginal state of the circuit, a means to turn off the thyristor D isnecessary. To meet this requirement, a manual switch is usually providedfor short-circuiting the anode and cathode of the thyristor D,,,. Themanual operation of the switch to return the circuit to the initialstate, however, is very troublesome in actual use. Also, absoluteprotection of the circuit cannot be expected.

FIG. 3 shows a constant-voltage circuit according to the invention,which eliminates the above drawbacks while utilizing the fundamentalexcellent features of the thyristor. In the Figure, reference symbol Tr;designates a transistor for shortcircuiting the base of the transistorTr, to cut it off, symbol Tr, a transistor for controlling thetransistor Tr symbol D, a level-shift diode, symbol R, a load resistorfor the transistor Tr,, and symbol R, a base resistor for the transistorTn, which also serves as the load resistor for a thyristor D,,,. Thethyristor D is provided for the function of withholding theload current.The withholding period is determined by resistor R, and R a capacitor Cand a transistor Tr The operational principles underlying the inventionwill now be described with reference to FIG. 5, which shows potentialsat various points in the circuit of FIG. 3. Point A is common to theanode of the thyristor D and level-shift diode D point B is theconnection point between the collector of the transistor Tr, and thebase of the transistor Tr,. point D is the connection point between thecollector of the transistor Tr, and the base of the transistor Tr,,point E is at the emitter of the transistor Tr,, and point F is commonto the resistors R, and R, and the capacitor C,.

When the load current increases to reach a preset value. the voltagedrop across the resistor R is increased to trigger the thyristor D,,,,thus reducing the potential at point A. As a result, the transistor Tr,is triggered to increases the potential at point B. With increase in thepotential at point B the transistor Tr is triggered to reduce thepotential at point D. As a result of the decrease of the potential atpoint D the emitter potential on the transistor Tr, at point E isreduced to reduce the output' current. Meanwhile, with increase in thepotential at point B the potential at point P begins to increase inaccordance with the time constant depending upon the resistor R andcapacitor C;,. When the terminal voltage across the capacitor C isincreased to increase the potential at point F to a predetermined value,the transistor Tr, is triggered to reduce the potential at point A so asto turn off the thyristor D,,,. The transistor Tr, carries current onlyfor a short period of time, and the initial value of the potential atpoint A is soon recovered. When the potential at point A is increasedthe transistor Tr, is triggered again, reducing the potential at point Bto turn off the transistor Tr;,, thus increasing the potential at pointD to thereby increase the potential at point E. The

potential at poinf F, on the other hand, is reduced since the potentialat point B is reduced.

FIG. 4 shows another embodiment of the invention.

In the Figure, reference symbol Tr, designates a transistor for shortcircuiting the base of the transistor Tr, to reduce the emitterpotential thereof, symbol Tr a transistor for controlling the transistorTr symbol D a thyristor, symbol R a load resistor for the transistorTr,, symbol D a level-shift diode, symbol R, a resistor for adjustingthe sensitivity of the transistor Tr, symbol R a resistor serving bothas the base resistor for the transistor Tr and as the load for thethyristor D,,,, symbol R, an overcurrent detection resistor, symbol La ameans to convert the current through the thyristor D into light, forinstance a lamp, and symbol CdS a photoconductive cell optically coupledwith the lamp La such that its resistance varies in accordance with thequantity of incident light, or more particularly, its resistance reduceswith increase in the quantity of incident light and increases withdecrease in the quantity of incident light. Point A is the anode pointat the thyristor D point B is the connection point between the collectorof the transistor Tr and the base of the transistor Tr point D is theconnection point between the collector of the transistor Tr and the baseof the transistor Tr,, and point E is the emitter point at thetransistor Tr,. Normally, the transistor Tr is off, transistor Tr on"and thyristor D off, and no current is flowing through the lamp La, sothat the resistance of the photoconductive cell CdS is high. When theload current exceeds a predetermined value, the voltage drop across theresistor R is increased to trigger the thyristor D,,,. Upon triggeringof the thyristor D current is caused to pass through the lamp La tolight it. There is a certain delay time between the rushing of currentinto the lamp La and its lighting. Upon lighting of the lamp La thequantity of light incident on the photoconductive cell CdS is increasedto reduce the resistance thereof. There is also a slight time delaybetween the instant of the incidence of light on the photoconductivecell CdS and the instance of the corresponding variation of theresistance of the photoconductive cell CdS. The interrelation among thelamp current, the intensity of light from the lamp and the resistance ofthe photoconductive cell is shown in FIG. 6. Meanwhile, upon triggeringof the thyristor D the transistor Tr, is triggered to increase thepotential at point B. With the increase of the potential at point B thetransistor Tr is triggered to decrease the potential at point D, therebydecreasing the potential at point E to reduce the load current. With thedecrease in the resistance of the photoconductive cell CdS the currenttherethrough is increased to decrease the current carried by thethyristor D,,,. When the current through the thyristor D becomes lessthan a predetermined holding current,the thyristor D is turned off.Therefore, it is desirable to preset the lamp La and the photoconductivecell CdS such that the resistance of the photoconductive cell CdS issufficiently low even with as low current through the thyristor D asequal to the predetermined holding current. When the thyristor D is cutoff, current through the lamp La ceases, so that the lamp La is turnedoff to stop the irradiation of the photoconductive cell CdS. As aresult, the resistance of the photoconductive cell CdS increases toincrease the potential at point A. With the increase of the potential atpoint A the transistor Tr, is triggered to decrease the potential atpoint B, thereby turning off the transistor Tr to increase the potentialat point D. Thus, the initial condition of the circuit is recovered. Inthis embodiment, the time delay involved between the rushing of currentinto the lamp La and the lighting thereof and between the instant ofirradiation of the photoconductive cell CdS and the instant of change ofthe resistance thereof is effectively utilized. in FIG. 5, 7 representsthe period, during which period the protective action is beingperformed. The potential at points A to E in this embodiment varies in asimilar way to that shown in FIG. 5. As has been described in theconnection with the foregoing embodiments of H68. 3 and 4, according tothe invention it is possible to automate the operation of recovering thenormal state of the overcurrentproof constant-voltage circuit using athyristor, which has heretofore been manually attained by means of areducing the burden on the control transistor at the time of its action,which 15 the feature of the thyristor, and the function of automaticallyrecovering its normal state.

What is claimed is:

l. A power source circuit comprising a current control means to beconnected in series with a load, an overcurrent preventing meansconnected to said current control means and having a thyristor, saidovercurrent preventing means comprising a means for recovering thenormal state of said thyristor after the lapse of a predetermined timeinterval fol lowing the actuation of said overcurrent preventing means,whereby when current supplied to said load exceeds a predetermined valuesaid overcurrent preventing means is actuated to operate said currentcontrol means so as to temporarily cut the current supply to said loadfor a constant period of time until the normal state of said currentcontrol means is restored by said overcurrent preventing means, andwherein said means for recovering the normal state of said thyristor insaid overcurrent preventing means consists of a circuit having apredetermined time constant for short circuiting said thyristor to turnoff said thyristor after the lapse of a predetermined time intervalafter the terminal voltage across said thyristor is changed to triggersaid thyristor.

2. A power source circuit comprising a current control means to beconnected in series with a load, an overcurrent preventing meansconnected to said current control means and having a thyristor, saidovercurrent preventing means comprising a means for recovering thenormal state of said thyristor after the lapse of a predetermined timeinterval following the actuation of said overcurrent preventing means,whereby when current supplied to said load exceeds a predetermined valuesaid overcurrent preventing means is actuated to operate said currentcontrol means so as to temporarily cut the current supply to said loadfor a constant period of time until the normal state of said currentcontrol means is restored by said overcurrent preventing means, andwherein said circuit having a predetermined time constant includes aresistor and a capacitor connected in series, on which circuit there isimpressed a voltage corresponding to the terminal voltage across saidthyristor in said overcurrent preventing means, and a transistorconnected in parallel with said thyristor for detecting the terminalvoltage across said capacitor, whereby when said thyristor is triggereda voltage is impressed across said resistor and the capacitor connectedin series, and when said transistor is triggered said transistor shortcircuits said thyristor for a constant time determined by the timeconstant of said series circuit.

3. A power source circuit according to claim I, wherein said circuithaving a predetermined time constant includes a transistor for detectingthe terminal voltage drop across said thyristor in said overcurrentpreventing means, a series connection of a resistor and a capacitor,said series connection being connected between the collector and emitterof said transistor, and a transistor connected in parallel with saidthyristor and cutting off said thyristor by detecting a voltage built upacross said capacitor.

4. A power source circuit according to claim 1, wherein said circuithaving a predetermined time constant includes a light source connectedin series with said thyristor in said overcurrent preventing means, anda photoconductive element connected in parallel with said thyristor andoptically coupled with said light source and delay in the operation ofsaid photoconductive element with respect to the operation of said lightsource is utilized.

5. A power source circuit according to claim 4, wherein said lightsource is a lamp and said photoconductive element is a CdS cell.

1. A power source circuit comprising a current control means to beconnected in series with a load, an overcurrent preventing meansconnected to said current control means and having a thyristor, saidovercurrent preventing means comprising a means for reCovering thenormal state of said thyristor after the lapse of a predetermined timeinterval following the actuation of said overcurrent preventing means,whereby when current supplied to said load exceeds a predetermined valuesaid overcurrent preventing means is actuated to operate said currentcontrol means so as to temporarily cut the current supply to said loadfor a constant period of time until the normal state of said currentcontrol means is restored by said overcurrent preventing means, andwherein said means for recovering the normal state of said thyristor insaid overcurrent preventing means consists of a circuit having apredetermined time constant for short circuiting said thyristor to turnoff said thyristor after the lapse of a predetermined time intervalafter the terminal voltage across said thyristor is changed to triggersaid thyristor.
 2. A power source circuit comprising a current controlmeans to be connected in series with a load, an overcurrent preventingmeans connected to said current control means and having a thyristor,said overcurrent preventing means comprising a means for recovering thenormal state of said thyristor after the lapse of a predetermined timeinterval following the actuation of said overcurrent preventing means,whereby when current supplied to said load exceeds a predetermined valuesaid overcurrent preventing means is actuated to operate said currentcontrol means so as to temporarily cut the current supply to said loadfor a constant period of time until the normal state of said currentcontrol means is restored by said overcurrent preventing means, andwherein said circuit having a predetermined time constant includes aresistor and a capacitor connected in series, on which circuit there isimpressed a voltage corresponding to the terminal voltage across saidthyristor in said overcurrent preventing means, and a transistorconnected in parallel with said thyristor for detecting the terminalvoltage across said capacitor, whereby when said thyristor is triggereda voltage is impressed across said resistor and the capacitor connectedin series, and when said transistor is triggered said transistor shortcircuits said thyristor for a constant time determined by the timeconstant of said series circuit.
 3. A power source circuit according toclaim 1, wherein said circuit having a predetermined time constantincludes a transistor for detecting the terminal voltage drop acrosssaid thyristor in said overcurrent preventing means, a series connectionof a resistor and a capacitor, said series connection being connectedbetween the collector and emitter of said transistor, and a transistorconnected in parallel with said thyristor and cutting off said thyristorby detecting a voltage built up across said capacitor.
 4. A power sourcecircuit according to claim 1, wherein said circuit having apredetermined time constant includes a light source connected in serieswith said thyristor in said overcurrent preventing means, and aphotoconductive element connected in parallel with said thyristor andoptically coupled with said light source and delay in the operation ofsaid photoconductive element with respect to the operation of said lightsource is utilized.
 5. A power source circuit according to claim 4,wherein said light source is a lamp and said photoconductive element isa CdS cell.